Recently, for the radiator, evaporator and condenser of air-conditioner, etc. as the heat-exchanger for motorcars, Al or Al alloys have become to be used broadly for the lightening in weight. In general, for the manufacture of aluminum heat-exchanger, a brazing sheet laminated with brazing alloy having a lower melting point than the core material, for example, Al-Si alloy or Al-Si-Mg alloy onto one or both sides of core material comprising Al or Al alloy is used. This is combined with Al component, for example, extruded multihole tube to permit the mass production by brazing.
For such aluminum heat-exchangers manufactured by the heating for brazing, following methods are used to assure the pitting corrosion resistance.
(1) Chloride flux containing ZnCl.sub.2 is used for the heating for brazing and Zn is allowed to deposit and diffuse onto the surface of Al components simultaneously with the brazing. The pitting corrosion of Al components is prevented through the sacrificial action of said diffuse layer. This method is excellent in the pitting corrosion resistance and utilized mainly for the condenser or cooler.
(2) Such elements as make Al or Al alloys base electrochemically when adding Zn, Sn, In, etc. are added to fin material or brazing material. Through the sacrificial action thereof, the pitting corrosion of Al components constructing the passage of refrigerant etc. is prevented. This method is utilized mainly for the condenser, evaporator, radiator, etc.
(3) Al-Zn alloy, Al-Zn-Mg alloy or pure Al is used as a skin material and this is cladded to various core materials to prevent the generation of pitting corrosion of core material through the sacrificial action of skin material. This method is utilized for the improvement in the pitting corrosion resistance of tube (seam welded tube), header, etc. of radiator, in particular, for the water side thereof.
(4) To the extruded multihole tube used for the condenser tube, a layer covered with Zn is provided beforehand by flame spray coating with Zn, zincate treatment, Zn plating, etc. Then, the diffuse layer of Zn is formed by the heating for brazing to prevent the pitting corrosion of extruded multihole tube.
All of conventional methods above to assure the corrosion resistance of aluminum heat-exchanger have following problems and the improvement therein is desired earnestly.
In the method (1) above, the post-treatments such as washing with water etc. becomes necessary because of the occurrence of corrosive flux residue and the production cost including the effluent treatment etc. accompanied with this becomes high. In the method (2) above, there is a restriction in the range of corrosion resistance of fin material and the effect of application thereof does not extend all over the core of heat-exchanger. In the method of adding to brazing material, the sacrificial layer cannot be formed sufficiently due to the melting of brazing material and, inversely, at a portion where the brazing material builds up, the sacrificial layer often comes into the core material deeply together with the diffusion of brazing material to lower the pitting corrosion resistance. In the method (3) above, the sacrificial layer must be cladded beforehand and the application is difficult to the extruded multihole tube etc., though the production is possible with the brazing sheet. Also, in the method (4) above, the surface of Al component becomes heterogeneous by the treatment with Zn resulting in the problems such as dropping out etc. due to the bending etc.
With regard to the method (1), a brazing method used the non-hygroscopic and noncorrosive fluoride flux has been developed thereafter. In this method, the eutectic composition of, for example, KAlF.sub.4 -K.sub.3 AlF.sub.6 is used for the flux and the brazing is performed by heating to about 600.degree. C. in the furnace, where the dew point is controlled not higher than -40.degree. C. and the partial pressure of O.sub.2 is controlled not more than 1000 ppm, introducing the inert gas, mainly N.sub.2 (hereinafter, such brazing method as this is referred to as NB method). Here, the washing after the brazing is unnecessary.
For example, in the case of the condenser of aluminum heat-exchanger by NB method, the extruded multihole tube (hereinafter, abbreviated as tube material) (1) processed with bender in a serpentine shape as shown in FIG. 1 (A) and the fin material (2) corrugated as shown in FIG. 1 (B) are assembled as shown in FIG. 1 (C). After fitted the unions (3) and (3') to the inlet and outlet of the refrigerant in tube material (1), respectively, this is fixed with pressing-down jigs (4) to make the core (5). Following the washing of said core, fluoride type flux is coated all over it and then this is fed to the brazing furnace to preheat and heat according to the temperature distribution curve shown in FIG. 3 (B). Thus, the fin material and the tube material are brazed and united.
For the fin material, the brazing sheet (thickness: 0.16 mm) laminated with JIS 4343 Al-Si alloy as a brazing material onto both sides of the core material comprising JIS 3003+1% Zn alloy is used. However, resulting from the traveling of motorcars in the areas of salt damage, the improvement in the exterior pitting corrosion resistance of said heat-exchanger has become an important problem recently. Particularly, in the NB method aforementioned, not only the use of sacrificial fin but also the corrosion-resistant treatment of tube material itself as below have become to be made.
(1) By submitting the tube material to the zincate treatment before brazing, Zn is allowed to deposit onto the surface of tube material and, by the heating for brazing, Zn is allowed to diffuse into tube material.
(2) By adding Zn to the fluoride flux, Zn is allowed to diffuse from flux into tube material upon heating for brazing.
However, the zincate treatment before brazing brings about high cost and, at the same time, since alkali solution is used for the zincate treatment of tube material, the invasion of the solution into tube material must be prevented resulting in many difficulties in the operation.
Moreover, in the method adding Zn to the flux, low concentration flux of about 10% is used satisfactorily in the case of fluoride flux due to the strong activity by itself on the contrary to the use of high concentration flux of 50 to 60% in the case of chloride flux. As a result, large amounts of Zn cannot be supplied and desired amount of Zn cannot be allowed to diffuse all over the surface.
On the other hand, a method is shown in Japanese Patent Publication No. Sho 59-31588, wherein the vapour of Zn is blown onto the surface of extruded material of Al to form a layer covered with Zn, this is allowed to diffuse onto the surface of extruded material of Al by the heatings for brazing etc., and the corrosion resistance is improved through the sacrificial effect of surface layer. The generation of the vapour of Zn in this case is performed in such a way that the gas-introductory pipe is inserted into the melt of Zn kept at 550.degree. C., the vapour of Zn is allowed to disperse into N.sub.2 gas being a carrier by supplying N.sub.2 gas to bubble, and the vapour of Zn is blown onto the surface of extrusion-moulded material of Al via the passage kept hot to form the layer covered with Zn on the surface of extrusion-moulded material of Al. The thickness of the layer covered with Zn is adjusted by the extruing velocity of extrusion-moulded material and the supplying amount of gas.
However, with the extrusion-moulded material of Al, the blowing of the vapour of Zn is easy inside the extruded material, but, on the outside, the vapour of Zn ends to scatter around and further the oxidation proceeds in the air. Therefore, it is difficult to provide the uniform layer of Zn for a short time. Moreover, with the aluminum heat-exchanger used the brazing sheet, the blowing of the vapour of Zn is difficult at the time of manufacturing the brazing sheet material because of the large width of plate to make the application impossible. Furthermore, since N.sub.2 gas is bubbled as a method of generating the vapour of Zn, the facility for supplying N.sub.2 gas under high pressure, the furnace for retaining the melt of Zn and the pipings are needed.